In the chemical industry for carrying out chemical operations that require the contacting of different phases that meet mostly in a counterflow e.g. such as absorption, distillation, etc. columns are used which can comprise several stages of plates, random or stacked packings or regular tower packings.
The efficiency of a phase-contacting assembly can be characterized by a few number of operational parameters e.g. by the quantity of material that can be processed in a unity volume of column or by the pressure drop that relates to a theoretical stage, etc. Regularly arranged packing structures can generally process higher quantities of material per unity volume of column with smaller pressure loss per theoretical stage than other packing bodies or trays can. Owing to such properties orderly arranged packing structures are especially appropriate for use under vacuum.
In tray columns in which the contact of different phases takes place in a stagewise manner, the expansion of the interfacial area, which decisively influences the mass transfer, occurs mainly on account of the flow energy of the gas phase i.e. it is associated with a higher pressure drop. The gas overcomes the hydrostatic pressure of the liquid on the plates, penetrates therethrough and it raises a fraction of the liquid in the form of a foam or droplets in the space between the plates, and the material transfer takes place through the so obtained free liquid surface i.e. at the phase boundary surface. For avoiding direct liquid transport only a portion of the space between the plates can thus be utilised which is connected with the less effective utilization of the column.
In packed columns in which there is a continuous contact between the phases, the liquid is flowing down along the mantle surfaces of the packing bodies, while the gas phase flows up through the space between the bodies which space defines free cross sections of flow. This free cross section can vary from layer to layer and it varies sometimes even within a layer as well. The interfacial area is constituted primarily by the wetted surface of the bodies. A portion of the flow energy of the gas phase gets lost as a friction loss on the surface of the bodies and other parts of this energy get lost mainly due to shape resistance and to swirling losses in the zig-zag channels between the bodies having varying cross sections.
In case of wetted packing bodies a portion of the increased loss of flow energy of the gas phase is utilised to wet the surface of the packing more effectively, whereby an increased interfacial area is obtained. In the smaller gas flow channels the gas retains more liquid, while in the larger passage channels the liquid can flow down in narrow streams without forming a film. This results first in the forward mixing of parts of the liquid phase i.e. both the flow profile and concentration profile will differ from the ideal piston-like flow and on the other hand in excess pressure drop. The increase of the liquid volume being jammed in contracting channel sections results in further increase in pressure losses and might cause local flooding. If the random packing has an inappropriate structure, the free cross section of flow can be decreased to such extents in one or more horizontal planes of the column that might cause the formation of `liquid plugs`.
The use of ordered or regular packing structures can overcome most of such drawbacks of randomly packed columns. Ordered packing structures consist of sheets or ribbons arranged regularly in a side by side relationship.
Ordered packing structures comprise passage channels which can be considered to represent the elementary units thereof. The passage channels are open both at their upper and lower ends. In a counterflow operation the liquid phase enters the passage channels from above, while the gas enters from below. The liquid flows down on the surface of the sheets defining the sides of the passage channels, while the gas flows up in the space defined between the channel walls. The passage channels can also be open at portions of their sides. An ordered assembly of the passage channels forms generally a packing unit. In a column an ordered packing comprises a number of packing units placed above each other (in which adjacent units can be angularly displaced relative to each other).
The side walls of different kinds of ordered packing units are made generally by arranging appropriate corrugated sheets beside each other that can be made by folding, pressing or by other ways from continuous or apertured sheets.
Owing to the uniform structure of the passage channels in ordered packing units the free cross section of flow is uniform in different horizontal layers as well as within the layers. This property decreases the liquid hold-up and the pressure losses and provides flow and concentration profiles which are close to the ideal piston-like profile.
In U.S. Pat. Nos. 3.415.502 and 2.940.168 packing structures are shown that comprise passage channels open at one side and which each other at their open sides being then in mutually opposing relationship and the axes of such channels close respective angles with each other as well as with the axis of the column. The liquid enters the passage channels in the form of droplets or of jet streams, then abuts the slanted walls of the channels and gets spread thereon and flows down in directions which are divergent within every pair of the passage channels. The gas (vapor) enters the passage channels from below and during its oblique upstream the flow of a channel meets through the open side of the channel with the oppositely inclined stream of the neigbouring channel that crosses the first channel, and at the meeting zones swirling occurs due to mutual friction of the streams. The oblique arrangement of the passage channels facilitate the maxing of the gas and liquid phases in radial direction, but the increased flow resistance due to zig-zag flow is connected with significant pressure losses.
The swirl-formation and the increased flow resistance result in excess pressure losses that cannot create excessive interfacial area.
In U.S. Pat. No. 3.227.429 an ordered packing structure is described which has passage channels extending parallel both to each other and to the axis of the column. These passage channels have closed vertical side walls. In each layer the passage channels are offset relative to each other by a half period. Owing to such design the two phases entering and leaving the passage channels in counterflow will be united and separated in an alternating manner without the need of using any additional deflection means. This design has the drawback that the radial distribution is smaller than required and the downwardly flowing liquid phase cannot impinge a slanted wall that would facilitate its spreading, thus the liquid can flow through the passage channel in the form of drops or of a wider or narrower jet which flow is associated first with a smaller proportion of the effectively damped surface and second with the occurrence of unwanted forward mixing. The packing structure described in the Czechoslowakian patent 206.918 has similar properties.